A Ventromedial Prefrontal-to-Lateral Entorhinal Cortex Pathway Modulates the Gain of Behavioral Responding During Threat

BACKGROUND

The ability to correctly associate cues and contexts with threat is critical for survival, and the inability to do so can result in threat-related disorders such as posttraumatic stress disorder. The prefrontal cortex (PFC) and hippocampus are well known to play critical roles in cued and contextual threat memory processing. However, the circuits that mediate prefrontal-hippocampal modulation of context discrimination during cued threat processing are less understood. Here, we demonstrate the role of a previously unexplored projection from the ventromedial region of PFC (vmPFC) to the lateral entorhinal cortex (LEC) in modulating the gain of behavior in response to contextual information during threat retrieval and encoding.

METHODS

We used optogenetics followed by in vivo calcium imaging in male C57/B6J mice to manipulate and monitor vmPFC-LEC activity in response to threat-associated cues in different contexts. We then investigated the inputs to, and outputs from, vmPFC-LEC cells using Rabies tracing and channelrhodopsin-assisted electrophysiology.

RESULTS

vmPFC-LEC cells flexibly and bidirectionally shaped behavior during threat expression, shaping sensitivity to contextual information to increase or decrease the gain of behavioral output in response to a threatening or neutral context, respectively.

CONCLUSIONS

Glutamatergic vmPFC-LEC cells are key players in behavioral gain control in response to contextual information during threat processing and may provide a future target for intervention in threat-based disorders.

Resolving equivocal gain modulation of corticospinal excitability

The ratio between the input and output of neuronal populations, usually referred to as gain modulation, is rhythmically modulated along the oscillatory cycle. Previous research on spinal neurons, however, revealed contradictory findings: both uni- and bimodal patterns of increased responsiveness for synaptic input have been proposed for the oscillatory beta rhythm. In this study, we compared previous approaches of phase estimation directly on simulated data and empirically tested the corresponding predictions in healthy males and females. We applied single-pulse transcranial magnetic stimulation over the primary motor cortex at rest, and assessed the spinal output generated by this input. Specifically, the peak-to-peak amplitude of the motor evoked potential in the contralateral forearm was estimated as a function of the EMG phase at which the stimulus was applied. The findings indicated that human spinal neurons adhere to a unimodal pattern of increased responsiveness, and suggest that the rising phase of the upper beta band maximizes gain modulation. Importantly, a bimodal pattern of increased responsiveness was shown to result in an artifact during data analysis and filtering. This observation of invalid preprocessing could be generalized to other frequency bands (i.e., delta, theta, alpha, and gamma), different task conditions (i.e., voluntary muscle contraction), and EEG-based phase estimations. Appropriate analysis algorithms, such as broad-band filtering, enable us to accurately determine gain modulation of neuronal populations and to avoid erroneous phase estimations. This may facilitate novel phase-specific interventions for targeted neuromodulation.

The role of astrocytes in place cell formation: A computational modeling study

Place cells develop spatially-tuned receptive fields during the early stages of novel environment exploration. The generative mechanism underlying these spatially-selective responses remains largely elusive, but has been associated with theta rhythmicity. An important factor implicating the transformation of silent cells to place cells is a spatially-uniform depolarization that is mediated by a persistent sodium current. This neuronal current is modulated by extracellular calcium concentration, which, in turn, is actively controlled by astrocytes. However, there is no established relationship between the neuronal depolarization and astrocytic activity. To consider this link, we designed a bioplausible computational model of a neuronal-astrocytic network, where astrocytes induced the transient emergence of place fields in silent cells, and accelerated the plasticity-induced consolidation of place cells. Interestingly, theta oscillations emerged naturally at the network level, resulting from the astrocytic modulation of subcellular neuronal properties. Our results suggest that astrocytes participate in spatial mapping and exploration, and further highlight the computational roles of these cells in the brain.

Interaction between the pulmonary stretch receptor and pontine control of expiratory duration

Medial parabrachial nucleus (mPBN) neuronal activity plays a key role in controlling expiratory (E)-duration (TE). Pulmonary stretch receptor (PSR) activity during the E-phase prolongs TE. The aims of this study were to characterize the interaction between the PSR and mPBN control of TE and underlying mechanisms. Decerebrated mechanically ventilated dogs were studied. The mPBN subregion was activated by electrical stimulation via bipolar microelectrode. PSR afferents were activated by low-level currents applied to the transected central vagus nerve. Both stimulus-frequency patterns during the E-phase were synchronized to the phrenic neurogram; TE was measured. A functional mathematical model for the control of TE and extracellular recordings from neurons in the preBötzinger/Bötzinger complex (preBC/BC) were used to understand mechanisms. Findings show that the mPBN gain-modulates, via attenuation, the PSR-mediated reflex. The model suggested functional sites for attenuation and neuronal data suggested correlates. The PSR- and PB-inputs appear to interact on E-decrementing neurons, which synaptically inhibit pre-I neurons, delaying the onset of the next I-phase.

Transient neocortical gamma oscillations induced by neuronal response modulation

In this paper a mean field model of spatio-temporal electroencephalographic activity in the neocortex is used to computationally study the emergence of neocortical gamma oscillations as a result of neuronal response modulation. It is shown using a numerical bifurcation analysis that gamma oscillations emerge robustly in the solutions of the model and transition to beta oscillations through coordinated modulation of the responsiveness of inhibitory and excitatory neuronal populations. The spatio-temporal pattern of the propagation of these oscillations across the neocortex is illustrated by solving the equations of the model using a finite element software package. Thereby, it is shown that the gamma oscillations remain localized to the regions of neuronal modulation. Moreover, it is discussed that the inherent spatial averaging effect of commonly used electrocortical measurement techniques can significantly alter the amplitude and pattern of fast oscillations in neocortical recordings, and hence can potentially affect physiological interpretations of these recordings.